Flatiron School Presents – The Multipeer Connectivity Framework (Lattice)

Another Flatiron student and I were interested in the Multipeer Connectivity Framework in iOS, so we made a chat app called Lattice that uses multipeer in the event of a natural disaster as a demonstration of the technology.

I’m going to talk about how the basics of multipeer connectivity work.

The Multipeer Connectivity (MC) Framework

What does it do?

Multipeer allows devices to connect with others nearby without the internet.

How does it work?

Multipeer Connectivity is an implementation of mesh networking, creating a local network between devices within Wifi or Bluetooth range. Not only are new devices connected with devices within range but also with all devices that those devices are connected to, even if the destination device is not within range of the sender.

That’s the real power of mesh networks. If permitted, devices can send messages relayed through other devices.

How might this be useful?

  1. Mesh networks allow communication when internet infrastructure is damaged (such as natural disasters).
  2. Mesh networks allow communication when internet access is restricted by governments, seen in the protests of Hong Kong.
  3. Mesh networks allow communication in internet-less situation (camping trips, road trips, subway commutes, and foreign vacations).

Lattice – Demo Project

Before we dive into the nitty gritty, I’ve placed Lattice on Github, so you can see exactly how the snippets below are used.

Setting up the MCManager

We have a MCManager class, similar to an API manager or data store for Core Data. Four properties need to be set up.


_peerID = [[MCPeerID alloc] initWithDisplayName:displayName];

This is the name of your device.


_session = [[MCSession alloc] initWithPeer:self.peerID securityIdentity:nil encryptionPreference:MCEncryptionNone];
_session.delegate = self;

This is the session that your device will start. Think of a session like a chatroom.


_advertiser = [[MCNearbyServiceAdvertiser alloc] initWithPeer:self.peerID discoveryInfo:elements serviceType:LatticeServiceType];
self.advertiser.delegate = self;
[self.advertiser startAdvertisingPeer];

This adverser tells nearby browsers that it is looking to connect with a particular serviceType. DiscoveryInfo is a dictionary that the browser will receive when it finds this advertiser.


_browser = [[MCNearbyServiceBrowser alloc] initWithPeer:self.peerID serviceType:LatticeServiceType];
self.browser.delegate = self;
[self.browser startBrowsingForPeers];

This browser looks for advertisers of a particular service type to connect to.

Connecting to Other Devices

Lattice, the demo app, will have an advertiser and browser running at the same time on each device.

Browser Finds Peer, Invites Peer

- (void)browser:(MCNearbyServiceBrowser *)browser foundPeer:(MCPeerID *)peerID withDiscoveryInfo:(NSDictionary *)info
   [self.browser invitePeer:peerID toSession:self.session withContext:nil timeout:10]

When a browser finds an advertiser, it can parse the discoveryInfo that the advertiser set when it was created. In this case, we want the devices to connect automatically, so the browser sends an invitation to connect to the browser’s session.

Advertiser Accepts Invitation

- (void)advertiser:(MCNearbyServiceAdvertiser *)advertiser didReceiveInvitationFromPeer:(MCPeerID *)peerID withContext:(NSData *)context invitationHandler:(void (^)(BOOL, MCSession *))invitationHandler
    invitationHandler(YES, self.session); 

Since we want the advertiser to connect automatically, the advertiser will reply YES to the invitation.

Session Confirms Connection

- (void)session:(MCSession *)session peer:(MCPeerID *)peerID didChangeState:(MCSessionState)state

We can indeed confirm that the two devices are connected with this delegate method from the session. The session will tells us that the peer is now in the connected state.

Sending and Receiving Messages

Sending From the View Controller

[self.multipeerManager.session sendData:messageData toPeers:self.multipeerManager.session.connectedPeers withMode:MCSessionSendDataUnreliable error:&error];

Messages are sent from the session to the peers in the session.

Receiving In the MCManager

- (void)session:(MCSession *)session didReceiveData:(NSData *)data fromPeer:(MCPeerID *)peerID
    //Kicks off a notification to the View Controller 

Messages are received by the session on the other devices. In Lattice, we triggered a notification with each message received.

Catching the Notification In the View Controller

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(peerDidReceiveDataWithNotification:) name:@"MCDidReceiveDataNotification" object:nil];
- (void)peerDidReceiveDataWithNotification:(NSNotification *)notification
    NSData *messageData = notification.userInfo[@"data"];
    NSArray *messages = [NSKeyedUnarchiver unarchiveObjectWithData:messageData];
    [self.demoData.messages addObject:messages[0]];

The notification was handled by the view controller to display the new message.


iOS only

Multipeer Connectivity is a iOS framework.

Range can be limited on Bluetooth

We found it to be about 60 – 100 feet.

Downloading the app requires the internet?!

There should be a way for the app to spread virally with no internet.

Limited usefulness in zombie apocalypse

This would be great for finding survivors nearby without having to go room by room through scary dark buildings. During night when the zombies are outside, the bluetooth signal may not be long enough to communicate between buildings.

Special Thanks

Peter Fennema for showing how to connect via multipeer automatically

Jesse Squires for the messaging user interface: JSQMessagesViewController

Flatiron School Week 7 – BocceGame built with UIDynamicAnimator

UIKit Dynamics

In my spare time, I wanted to look at animating physics collisions so I found UI Dynamics. It’s a part of UIKit that has some build-in functionality to animate physics based interactions. It’s generally not used for games, but rather for transitions and cool UI tricks.

A bocce game is a simple way to demonstrate this functionality. In bocce, there’s a target ball called the jack and two teams try to get their ball as close to the jack as possible. The team gets points for the number of balls that are closer to the jack than the nearest opponent ball.

Bocce Game Demo

Github Repo

How to use UIDynamicAnimator

You have an animator.

Animators can add behaviors.

Behaviors can add items that they affect.

My animator

    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

One behavior is the UIDynamicItemBehavior behavior that allows the ball to continue moving after released by the hand with the add

self.linearVelocity = [[UIDynamicItemBehavior alloc] init];
[self.linearVelocity addLinearVelocity:velocity forItem:gesture.view];
[self.animator addBehavior:self.linearVelocity];

Another behavior is the collision behavior that collides with either a wall or another item.

//initialize collision behavior
self.collision = [[UICollisionBehavior alloc] init];
[self.animator addBehavior:self.collision];

How to make this game

Dragging a ball

Set up a UIPanGestureRecognizer that calls the dragged: method.

    UIPanGestureRecognizer *draggable = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(dragged:)];
    [self.currentBlock addGestureRecognizer:draggable];

The dragged method has the form:

- (void) dragged:(UIPanGestureRecognizer *)gesture
switch (gesture.state) {
case UIGestureRecognizerStateBegan:
//save the starting location

case UIGestureRecognizerStateChanged:
CGPoint translation = [gesture translationInView:self.view];
//calculate the new location with the starting location and the translation
//set the location to the new location
case UIGestureRecognizerStateEnded:
//figure out the velocity when the ball is released and add it to the item with the UIDynamicItemBehavior


Continue a ball moving after release

Right now, the ball would stop when I let go of it. To keep it going at the same speed, get the velocity in view from the gestureRecognizer and add it to the UIDynamicItemBehavior with addLinearVelocity: forItem:

//Inside the case UIGestureRecognizerStateEnded
CGPoint velocity = [gesture velocityInView:self.view];
[self.linearVelocity addLinearVelocity:velocity forItem:gesture.view];
[self.animator addBehavior:self.linearVelocity];
//necessary to make the motion smooth, otherwise it will be jumpy if you hold down the 
[self.animator updateItemUsingCurrentState:gesture.view];

Stopping the ball

The ball will keep going on for a long time, so I needed to stop it somehow.

Stop the ball by adding friction. In UIDynamicItemBehavior, friction is the resistive force between two objects. I wanted resistance to simulate air resistance. Angular resistance was needed to stop the balls from spinning too.

self.linearVelocity.resistance = 8;
self.linearVelocity.angularResistance = 8;

Only allow one animation to run at a time

I found that trying to flick a ball before another had finished moving was making a really jumpy animation, so checked if the animation is running with each gesture recognized and only let the new block move after the other was finished moving.

- (void) dragged:(UIPanGestureRecognizer *)gesture
    if (![self.animator isRunning]) {
        switch (gesture.state) {

Detecting when the ball was stopped

To find out when to make the next block, I needed to know when the ball had stopped moving. I used the

- (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator

method. It’s a delegate method so I had to set the

    self.animator.delegate = self;

Making the target ball bounce off the walls

I didn’t want the target ball to go off the screen, so I set the screen boundaries as walls,

[self.collision setTranslatesReferenceBoundsIntoBoundary:YES];

made the ball bound off all objects including boundaries (referenceBounds being those of self.view because the animator has self.view as the reference view),

[self.collision setCollisionMode:UICollisionBehaviorModeEverything];

and added another invisible wall, so that the ball would not end up on the top edge of the screen.

[self.collision addBoundaryWithIdentifier:@"end" fromPoint:CGPointMake(0, self.view.frame.size.height*0.1) toPoint:CGPointMake(self.view.frame.size.width, self.view.frame.size.height*0.1)];

Take the target ball out of collision

I wanted to make the target ball immovable, so I just had to remove it as an item of the collision.

[self.collision removeItem:self.currentBlock];

I also wanted future object to be able to go off the screen, so I removed the “end” boundary and the window boundaries.

[self.collision removeBoundaryWithIdentifier:@"end"];
[self.collision setTranslatesReferenceBoundsIntoBoundary:NO];

Ignoring most the call to dynamicAnimatorDidPause after a ball is made

After a new object is added, dynamicAnimatorDidPause is called, so I had to add a BOOL property justMadeBlock to ignore those calls.

Stop the moving block when it goes offscreen

If a ball was flicked too hard, then it would go off the screen and I would have to wait for it to stop before the next ball would load. To shorten that time, I made another boundary outside the edge of the screen and overwrote a delegate method to make the ball stop moving and not able to collide with other blocks.

[self.collision addBoundaryWithIdentifier:@"outOfBounds" forPath:[UIBezierPath bezierPathWithRect:CGRectMake(-90, -90, self.view.frame.size.width+180, self.view.frame.size.height+180)]];

Delegate method for the UICollisionBehavor

self.collision.collisionDelegate = self;
- (void)collisionBehavior:(UICollisionBehavior *)behavior beganContactForItem:(id<UIDynamicItem>)item withBoundaryIdentifier:(id<NSCopying>)identifier atPoint:(CGPoint)p
    if ([((NSString *)identifier) isEqualToString:@"outOfBounds"]) {
        //not allow it to move
        [self.linearVelocity removeItem:item];
        //not allow it to interact with future blocks
        [self.collision removeItem:item];

Calculate the nearest ball and marking the nearest ones

Once a colored ball had been thrown, there is now a winning ball (closest to the target).

I took all the balls on screen.

    NSArray *blocksOnScreen = [self.collision items];

Got their locations

Sorted the array of balls by location

And found the balls of the same color that were closest to the target ball

With those “winning balls”, I added a CALayer to them to mark them as winning.

The next to go would be the team further away from the target.

At the end of the game, a UIAlertController asks if you want to play again.

Adding indicators for the number of balls remaining per team

I used CAShapeLayers and added them to the self.view.layer. They would update as balls were made.

Adding a flashing turnIndicator

The alpha controls the opacity of the text label, so setAlpha:1.0 means make it appear.

UIViewAnimationOptionAutoreverse makes it disappear again.
UIViewAnimationOptionRepeat makes the animation keep going until you stop it.
UIViewAnimationOptionAllowUserInteraction allows you to remove the animation later on.

[UIView animateWithDuration:1.5 delay:0 options:UIViewAnimationOptionAutoreverse| UIViewAnimationOptionRepeat | UIViewAnimationOptionAllowUserInteraction  animations:^{
        [self.turnIndicator setAlpha:1.0];
    } completion:nil];

Stopping animations

When I tried to reset the game, setting all the properties back to nil worked well, except for stopping the turnIndicator animation.

There I found that the animation is in the layer of the animated object.

[self.turnIndicator.layer removeAllAnimations];

Choosing a more attractive color scheme

Adobe’s color wheel was really cool. I like pastel colors, so this is what I chose.

Notes to Self

Writing code takes way longer than expected

I didn’t think that it would take very long to make this, (just slap some blocks on a screen right?) but building features takes time and focus. Really quickly, seeing jumpy animations makes you want to fix them and that’s the 20% that takes the most time. Writing blogs about code also takes hours…

User testing is super important to do throughout

I knew how the game was played, but it wasn’t obvious to people I showed it to. I really wouldn’t have known if I hadn’t had someone use the game.

Don’t let sidetracked on super advanced features when the basics are not done

The flip side of asking for feedback is that people start giving you really good suggestions, but I think you have to finish one phase before hacking away at the next thing.

Hacking is fun with something quick, but it’s no fun with a big project

This is the second project that I’ve hacked to together. It’s been fun, but I really would have liked to have classes for the data model. This game was made over three weekends and I was finding that when I wanted to add new features, I had to recall the mental model that I had for the project. Instead I should have had a game class that remembered the model for me.

Flatiron School Week 4- Hacking together the TinderClone Swipe Feature

The Tinder Swipe

The central part of Tinder’s design is the swiping mechanism. The feature allows people to focus on one match at a time and make a decision before moving on to the next match.  Good for spending more time on matches and good for reducing the speed of data consumption.

I wanted to learn about animations, so this weekend I hacked together a prototype of the feature.


All the code can be found here along with a video demo. The code is very dirty, but that’s topic for another week.


Step 1. Look up how to do it.

The next step in my TinderClone project is to make the swipeable images. Richard Kim has a really cool blog entry about how to make this feature three different ways. Since I’m taking on this project to learn to code, I took the “build it from scratch” way and tried to follow Nimrod Gutman’s blog. I totally failed. Twice.

It turns out that I didn’t really understand how UIView and UIImageViews worked. So, I decided to play around with them.

Step 2. Read up on UIViews

I read thought through the Apple View Programming Guide. in particular, the view and window architecture. I didn’t really understand bounds very well, so I started an Xcode project called playingWithBounds, that ended up being my project.


  • Views are made on top of Core Animation layers (CALayer). Views have animation, but go to CALayer to one layer deeper for more control.
  • Subviews are arranged in an array in the superview.
  • UIContentMode is useful for sizing images. I played around with this later.
  • I thought the bounds property might have something to do with Box Sizing in CSS, but it’s much simpler. The bounds change the frame of the box.

UIView properties

  • frame- used for making objects. CGRectMake()
  • bounds- changes the size of the view, but not the center.
  • center- very important for moving objects around.
  • transform- used to rotate, move and scale objects. It’s the finishing touch. The object is drawn and then transformed. When an object is rotated, it remembers what it’s heigh and width were when it wasn’t rotated.
  • alpha- 1 is opaque and 0 is fully transparent. Great for
  • backgroundColor- useful for seeing where the rectangle is.

Step 3. Playing around with the UIView properties

I made single view project where I drew boxes and changed the bounds, and so on to see what would happen.

Step 4. Playing around with CGAffineTranform

When I got to Coordinate System Transformations, I started playing around with rotating and moving boxes.

I made a slider to rotate an UIImageView.

        CGRect slideFrame = CGRectMake(100, 400, 200, 20);
        self.pictureRotationAngle = [[UISlider alloc] initWithFrame:slideFrame];
        [self.view addSubview:self.pictureRotationAngle];
        self.pictureRotationAngle.minimumValue = 0;
        self.pictureRotationAngle.maximumValue = 2*M_PI;
        self.pictureRotationAngle.value = M_PI/2.0;
        [self.pictureRotationAngle addTarget:self action:@selector(rotateImage) forControlEvents:UIControlEventValueChanged];
        self.picture.transform = CGAffineTransformMakeRotation(self.pictureRotationAngle.value);

Step 5. Finding the Event Handling methods

Where other solutions mentioned at the top used UIGestureRecognizers, I imagined that these event handling methods could move my picture around too. They used UITouch objects.

I was touching my UIImageView and nothing was happening. Googling around, I found that UIImageViews are not user interactive by default. So, I had to set it.

    self.picture.userInteractionEnabled = YES;

I also set the

What is UITouch?

I didn’t know, so I built the first three methods and NSLogged the touch and event objects. They were very descriptive.

2014-10-27 18:58:26.919 TinderSwipeFeature[65292:2410541] <UITouch: 0x7fdf015533a0> phase: Moved tap count: 1 window: <UIWindow: 0x7fdf01409040; frame = (0 0; 375 667); gestureRecognizers = <NSArray: 0x7fdf01403730>; layer = <UIWindowLayer: 0x7fdf01414c50>> view: <UIImageView: 0x7fdf01684780; frame = (37.8671 64.9702; 402.124 402.124); transform = [0.99420459268393058, -0.10750454821160646, 0.10750454821160646, 0.99420459268393058, 0, 0]; clipsToBounds = YES; opaque = NO; layer = <CALayer: 0x7fdf01684ac0>> location in window: {256, 332} previous location in window: {255, 332} location in view: {192.38019379786252, 249.92073845470227} previous location in view: {191.38598920517859, 249.81323390649064}

UITouch has a lot of really useful info:

  • Size of the screen: 375 width and 667 points height.
    frame = (0 0; 375 667)
  • The view touched and it’s location
    view: <UIImageView: 0x7fdf01684780; frame = (37.8671 64.9702; 402.124 402.124)
  • Current touch location in view and window
    location in window: {256, 332} 
    location in view: {192.38019379786252, 249.92073845470227}
  • Previous touch location in view and window
    previous location in window: {255, 332}
    previous location in view: {191.38598920517859, 249.81323390649064}
  • Timestamp
    2014-10-27 18:58:26.919

I now had the velocity of touches and I just needed to update the view being touched with a new center value to move it.

That was the key to solving the problem.

I added the other Tinder style animations associated with swiping. All of these were based on using the x location of the touch divided by the width of the phone to calculate a factor that would modify the rotation of the image and the opacity of the overlaps.

Step 6. Playing with contentMode

When I put the picture in the UIImageView, it would not fit in the frame, so I had to set clipsToBounds to YES and change the contentMode from UIViewContentModeScaleToFill to UIViewContentModeScaleAspectFill. UIViewContentModeScaleAspectFit also works but you get white space on the sides.

    self.picture.contentMode = UIViewContentModeScaleAspectFill;
    self.picture.clipsToBounds = YES;

The other options such as UIViewContentModeTop maintain the size of the image, and align the listed edge to the corresponding edge in the box. For example,

  • UIViewContentModeTop aligns the image to the top of the UIImageView.
  • UIViewContentModeTopLeft aligns the image to the top and left of the UIImageView.

Step 7. Animation timing

For the touchesEnded method, I wanted to animate the image moving back into location.

[UIView animateWithDuration:0.4 delay:0 usingSpringWithDamping:0.6 initialSpringVelocity:0 options:UIViewAnimationOptionCurveEaseOut animations:^{
                ... reset image
            } completion:nil];

CGAffineTransformIdentity removes all transforms (rotations, scaling, translations).

Step 8. Animation for the picture flying off the screen or springing back

To animate the picture flying off to the side, I set a swipe threshold of 30% of the width.

if (differenceInTouchLocationX > aTouch.window.frame.size.width*0.2) {
            [UIView animateWithDuration:0.5 animations:^{
                viewTouched.center = CGPointMake(self.view.frame.size.width*2, currentLocation.y + 0.5 * speedY) ;
            NSLog(@"%@", aTouch);
        }else if (differenceInTouchLocationX < -aTouch.window.frame.size.width*0.3)
            [UIView animateWithDuration:0.5 animations:^{
                viewTouched.center = CGPointMake(-self.view.frame.size.width*2, currentLocation.y + 0.5 * speedY) ;
        else { 
             ... //springs back to original location

Step 9. Add the overlap images (Like and Nope)

self.like = [[UILabel alloc] initWithFrame:CGRectMake(20, 20, 100, 50)];
 [self.like setFont:[UIFont systemFontOfSize:100]];
 self.like.text = @"L I K E";
 self.like.adjustsFontSizeToFitWidth = YES;
 [self.like setBaselineAdjustment:UIBaselineAdjustmentAlignCenters];
 [self.like setTextColor:[UIColor colorWithRed:47/255.0f green:156/255.0f blue:28/255.0f alpha:1]];
// self.like.backgroundColor = [UIColor blueColor];
 self.like.transform = CGAffineTransformMakeRotation(-20* M_PI/180);
 self.like.alpha = 0;

Boy, I learned a lot by just reading the documentation and Googling. That was fun!